1. Field of the Invention
The present invention relates to an electric power controlling system for variable bit rate CDMA transmission which is applied to a mobile telephone system (cellular mobile telephone system) or the like using a direct spread code-division multiple accessing (DS-CDMA) system. More particularly, the invention relates to an electric power controlling system for variable bit rate CDMA transmission in which a plurality of bit rates are switched, and a mobile telephone system adopting the system.
2. Description of the Prior Arts
Hitherto, as a mobile telephone system of this kind adopting the direct spread code-division multiple accessing (DS-CDMA) system, IS-95 Standard of North American Standards TIA/EIA is widely known. In the IS-95 Standard, at the time of a reverse link from a mobile terminal as a transmitting side to a base station as a receiving side, one of four kinds of bit rates of 9.6 kbps, 4.8kbps, 2.4 kbps, and 1.2 kbps is selected for transmission. When 9.6 kbps is set to a basic rate and data is transmitted at bit rates of xc2xd, xc2xc, and xe2x85x9 of the basic rate, the variable rate transmission is executed in such a manner that transmission data is thinned to xc2xd, xc2xc, and xe2x85x9, respectively, and the resultant data is transmitted.
That is, one frame (20 msec) is divided into 16 slots each having an interval of 1.25 msec and, when data is transmitted at the basic rate of 9.6 kbps, the data is transmitted with all of 16 slots. In case of the bit rate of 2.4 kbps, 2.4 kbps, or 1.2 kbps, data is transmitted with only 8, 4, or 2 slots per one frame and transmission of other slots is stopped. In the method, the slot transmission is merely turned ON/OFF and a transmission electric power does not depend on the bit rate.
In the reverse link in the cellular mobile telephone system adopting the CDMA system, it is necessary to control a transmission electric power of a mobile terminal so that reception electric powers from respective mobile terminals such as a mobile telephone, and an information terminal are equalized. Particularly, in case of the variable bit rate system in which a bit rate of the mobile terminal dynamically changes, it is necessary to execute a control to equalize a reception electric power per information of one bit.
A control of the transmission electric power in the reverse link in OS-95 Standard will now be explained.
Since the base station cannot previously know a bit rate at which the mobile terminal transmits, the base station compares the reception electric power with a reference value for all slots and instructs the mobile terminal to increase or decrease the transmission electric power so that the reception electric power coincides with the reference value. The mobile terminal is controlled in accordance with only the instruction of the base station for the slots which have actually been transmitted and ignores the instruction to slots which have not been transmitted.
As mentioned above, in the transmission electric power control by the variable bit rate CDMA system, only the turn ON/OFF on a slot unit basis whereby the transmission electric power control is executed can be performed. Since the selection of bit rates is limited, an intra-frame interleave is restricted at a low bit rate, so that an effect of an error correction coding process is reduced. Therefore, since intervals of slots in which the reception electric power can be measured is widened, the control for the transmission electric power cannot follow a fast fading, so that its transmitting characteristics deteriorate. Since a burst transmission is executed, a hazard noise occurs. In this case, there is such a problem that an erroneous operation occurs due to the noise in, for example, a hearing aid or medical instruments located nearby.
On the other hand, in a forward link serving as an operation from the transmission of the base station to the reception of the mobile terminal of IS-95 Standard as well, the transmission in which one of the four kinds of bit rates of 9.6 kbps, 2.4 kbps, 2.4 kbps, and 1.2 kbps is selected can be similarly executed. However, it is different from a realizing method for the variable bit rate of the reverse link. In the forward link, the low bit rate is not realized by turning ON/OFF the transmission on a slot unit basis, namely, thinning in a manner similar to the reverse link. But, for example, in case of 4.8 kbps as shown in FIG. 1, the transmission electric power per one time is controlled to xc2xd instead of executing the transmission by repeating the same bit twice.
Similarly, in case of the bit rates of 2.4 kbps and 1.2 kbps, the electric powers per one time are set to xc2xc and xe2x85x9, respectively, instead of repeating the same bit four times and eight times, respectively. By the above-mentioned process, the transmission electric power per one bit can be held to be constant. In the variable bit rate processing method for the forward link as mentioned above, the problems involved in the reverse link as explained above, namely, a deterioration in characteristics due to the restriction of the interleave and hazard noise caused by with the burst transmission can be solved. However, since the transmission electric power is dynamically changed depending upon the bit rate in use, it is difficult to control the transmission electric power without previously knowing the bit rate so that the reception electric power per 1-bit information is equalized.
The reason why the realizing method for the variable bit rate for the forward link is different from that for the reverse link in IS-95 Standard is that the forward link uses a method in which it is unnecessary to execute a high-speed transmission electric power control and in which a deterioration in characteristics is small.
As such examples, xe2x80x9cSpread Spectrum Transmitting Methodxe2x80x9d disclosed in Japanese Patent Application Laid-Open No. 5-102943 (1993) and xe2x80x9cTransmission Bit Rate Discriminating Method and Apparatusxe2x80x9d disclosed in Japanese Patent Application Laid-Open No. 8-130535 (1996) are known. In Japanese Patent Application Laid-Open No. 5-102943 (1993), a variable bit rate transmitting and receiving method similar to that for the forward link in IS-95 Standard is disclosed but the above-described problems associated with the transmission electric power control are difficult to solve.
Further, in Japanese Patent Application Laid-Open No. 5-102943 (1993), an improvement in the determination of the transmission bit rate in the variable bit rate system is shown. However, since the bit rate can be determined only after completion of the reception of a reception signal of one frame (20 msec in IS-95 Standard), such transmission electric power control as executed for every slot (1.25 msec in IS-95 Standard)is not applicable. Similarly, as a method of determining a bit rate after completion of the reception of a reception signal of one frame, a method of comparing a likelihood when a CRC check is executed at a plurality of bit rates or when a maximum likelihood decode is performed to a convolution code by a Viterbi algorithm or the like is known. In this case, however, a timing of the transmission electric power cannot be processed.
In order to solve the problem of the variable bit rate transmitting system for the reverse link in IS-95 Standard, namely, a deterioration in transmitting characteristics caused by delays of the control for the interleave and control for the transmission electric power, there has been also proposed another variable bit rate transmission method for transmitting only part of slots without turning ON/OFF the transmission on a slot unit basis on which the transmission electric power controlled is executed in a manner similar to that in the reverse link of IS-95 Standard. For example, a method of transmitting a frame which is transmitted at a bit rate that is xc2xd of the basic rate by using the whole slot in such a manner that the transmission of only the former half of one slot is ON and that of the latter half is OFF.
In the variable bit rate transmitting method, since ON time per slot for transmission changes depending upon the bit rate, a measuring method for the reception of electric power is restricted. For example, when the reception electric power is measured during only intervals in which the transmission is ON at even the lowest bit rate, it is possible to prevent measurement of the reception electric power at intervals in which the transmission is OFF. However, there is a problem that the measurement interval is shortened, so that a measurement precision is deteriorated.
As mentioned above, in the conventional methods, in the variable bit rate for transmission wherein the transmission ON/OFF is effected on a slot unit basis as in the reverse link in IS-95 Standard, its transmitting characteristics are deteriorated.
In a variable bit rate transmitting method wherein changing a diffusion rate is changed in correspondence to the bit rate and simultaneously the transmission electric power is changed in inverse proportion to the diffusion rate, as in the forward link in IS-95 Standard, it is difficult to measure the reception electric power or a reception quality per 1-bit information. For example, in a method in which the diffusion rate is set to be twice as large as that of the basic rate and the transmission electric power is set to xc2xd of that of the basic rate in a frame in which data is transmitted at a bit rate of xc2xd of the basic rate.
In the variable bit rate transmission with only parts of the slots, for example, the method of transmitting a frame in which data is transmitted at a bit rate of xc2xd of the basic rate for the whole slot in such a manner that the transmission of only the former half of one slot is ON and that of the latter half is OFF, the measuring interval of the reception electric power is restricted, with the result that its measurement precision deteriorates.
The present invention has been made in consideration of the above points and it is the first object of the invention to provide a variable bit rate CDMA transmission electric power control system in which the transmission electric power is controlled at a high precision when it is applied to a variable bit rate transmitting method of transmitting data in such a manner that a transmission quality is not deteriorated, a spread factor is changed depending upon a bit as selected, and a transmission electric power is changed in inverse proportion to the spread factor and a variable bit rate transmission for transmitting only parts of slots depending upon a bit rate in use.
The second object of the invention is to provide a mobile telephone system adopting the CDMA transmission electric power controlling system according to the invention.
In order to accomplish the above objects, according to the invention, there is provided a variable bit rate CDMA transmission electric power controlling system for switching a plurality of bit rates in transmission wherein reception quality is estimated for transmission bit rates obtained from a reception base-band signal and a transmission electric power is controlled on the basis of the reception quality thus estimated.
According to the invention, the reception quality is estimated by measuring an Eb/Io value that is a ratio of a signal electric energy per bit Eb to an interference electric power per Hz Io value for the plurality of bit rates obtained from a reception base-band signal, obtaining a maximum value of the Eb/Io value, and a transmission electric power is controlled on the basis of the maximum value of the Eb/Io value.
Further, according to the invention, a variable bit rate CDMA transmission electric power controlling system comprises reception processing means for converting a reception signal to output a reception base-band signal; Eb/Io calculating means for calculating an Eb/Io value that is a ratio of a signal electric energy per bit Eb to an interference electric power per Hz Io for every bit rate in use; maximum value detecting means for detecting a maximum value of the Eb/Io values for all bit rates; comparing means for comparing the maximum Eb/Io value with a target Eb/Io value to produce a transmission electric power control bit instructing so that when the maximum Eb/Io value is large, a transmission electric power is decreased and, when the maximum Eb/Io value is small, the transmission electric power is increased; multiplexing means for multiplexing the transmission electric power control bit and transmission data to produce the resultant data; transmitting means for modulating and frequency converting multiplexed data and the transmission data for transmission; and demodulating means for demodulating and transmitting the reception base-band signal.
According to the invention, the variable bit rate CDMA transmission electric power controlling system further has as Eb/Io calculating means: a basic rate Eb/Io measuring unit for measuring an Eb/Io value for a basic rate; and a plurality of Eb/Io measuring units for calculating an Eb/Io value per one bit for remaining bit rates.
Further, according to the invention, in the variable bit rate CDMA transmission electric power controlling system, in each of the Eb/Io calculating means, maximum value detecting means, and comparing means, the transmission bit rate is changed with a frame having a time length of at least 10 msec to 80 msec as a unit and a process is executed by using a slot which is obtained by dividing one frame into a plurality of slots which have a time length of at least 0.5 msec to 2.5 msec as a unit.
According to the invention, in the variable bit rate CDMA transmission electric power control system, a spreading factor is changed depending upon the bit rate and the transmission electric power is changed in inverse proportion to the spreading factor.
Further, according to the invention, in the variable bit rate CDMA transmission electric power controlling system, the bit rate is changed within a range of one integer (N) of the basic rate by setting the maximum rate as a basic rate and, when the bit rate is equal to 1/N of the basic rate, the same data is repeated N times and is transmitted, the spreading factor is changed, and the transmission electric power is changed in inverse proportion to N.
According to the invention, in the variable bit rate CDMA transmission electric power controlling system, a time ratio in which the transmission is ON in the slot is changed in proportion to the bit rate.
Further, according to the invention, the variable bit rate CDMA transmission electric power controlling system has in order to execute the Eb/Io calculation: basic rate de-spreading unit for de-spreading the reception base-band signal; a plurality of symbol adding units for adding the number of transmission symbols repeated in correspondence to the bit rate; and a plurality of Eb/Io measuring units for measuring the Eb/Io values for the symbols added by the plurality of symbol adding units.
According to the invention, the variable bit rate CDMA transmission electric power controlling system has in order to perform the Eb/Io calculation: a de-spreading and inverse modulating unit for de-spreading the reception base-band signal and for inversely modulating the signal by using reception data, thereby eliminating a modulation component; a mean squared value unit for calculating a mean squared value of the signal from the de-spreading and inverse modulating unit in a slot interval; a plurality of mean squared value calculating units for obtaining a mean value from the signal from the de-spreading and inverse modulating unit in a transmission ON interval in the slot corresponding to the bit rate; and a plurality of Eb/Io measuring units for obtaining the Eb/Io value for every bit rate by using mean values from the mean squared value from the means squared value unit and the mean values from the plurality of mean value calculating units.
Further, according to the invention, the variable bit rate CDMA transmission electric power controlling system has as a plurality of Eb/Io measuring units: a basic rate Eb/Io measuring unit for measuring an Eb/Io value of the basic rate; a xc2xd rate Eb/Io measuring unit for measuring the Eb/Io value of xc2xd rate; a xc2xc rate Eb/Io measuring unit for measuring the Eb/Io value of xc2xc rate; and a xe2x85x9 rate Eb/Io measuring unit for measuring the Eb/Io value of xe2x85x9 rate.
According to the invention, the variable bit rate CDMA transmission electric power controlling system has as a plurality of symbol adding units: a 1-symbol adding unit for adding one symbol of a transmission repeated depending upon the bit rate from an output signal of the basic rate de-spreading unit; a 2-symbol adding unit for adding two symbols of a transmission repeated depending upon the bit rate; a 4-symbol adding unit for adding four symbols of a transmission repeated depending upon to the bit rate; and a 8-symbol adding unit for adding eight symbols of a transmission repeated depending upon the bit rate.
Further, according to the invention, the variable bit rate CDMA transmission electric power controlling system has as a plurality of mean value calculating units: a whole slot mean value unit for calculating a mean value of a whole slot interval; a xc2xd slot mean value unit for obtaining a xc2xd slot interval mean value; a xc2xc slot mean value unit for obtaining a xc2xc slot interval mean value; and a xe2x85x9 slot mean value unit for obtaining a xe2x85x9 slot interval mean value.
According to the invention, in the variable bit rate CDMA transmission electric power controlling system, the system is applied to a mobile telephone system using a direct spread code-division multiple accessing system.
With the construction according to the invention, the maximum value of the Eb/Io values measured for a plurality of bit rates obtained from the reception base-band signal is obtained and the transmission electric power control is executed.
Therefore, it is unnecessary to previously know the bit rate, so that it is possible to rapidly produce a transmission electric power control bit rate for every slot serving as a unit of the transmission electric power control. Consequently, a transmission quality is not deteriorated, a spreading factor is changed depending upon the bit rate, and when it is applied to a variable bit rate transmitting method of changing a transmission electric power in inverse proportion to a spreading factor and transmitting and a variable bit rate transmission for transmitting only parts of slots in correspondence to the bit rate, a transmission electric power control can be executed at a high precision.
In this case, in combination with the variable rate transmitting method of changing the spreading factor depending upon the bit rate, a transmitting efficiency is not deteriorated at even a low bit rate and it is possible to prevent an occurrence of hazard noise for, for example, a hearing aid or medical instruments. In combination with the variable rate transmitting method of varying ON time of a slot content depending upon the bit rate, measuring time for a reception quality can be extended, so that the precision of the transmission electric power control is improved.